18th. question.

Discussion in 'Electricians' Talk' started by seneca, Apr 26, 2019.

  1. Coloumb

    Coloumb Screwfix Select

    So your happy to install a s/f in a TT with tails > 3m in the DNO ctt without an earth knowing full well that neither may blow? What reg is that then?
     
  2. spinlondon

    spinlondon Screwfix Select

    I’m quite ‘Old School’ so I’d probably install a 100mA RCCB rather than a Switch Fuse.
    Of course now that we have to provide 30mA RCD protection to just about every circuit, I accept your point that we could just install a Switch Fuse.

    I install protective devices all the time knowing that they may never blow.
    Of course if a fault develops then hopefully they’ll blow, before something catches fire or someone’s injured.

    Not exactly sure how this discussion regarding protecting TT installations, helps your argument that the DNO’s fuse protects the consumer’s installation?
    To my mind, this just is just an example of how the DNO’s fuse is not there to protect the consumer’s installation.
     
  3. Coloumb

    Coloumb Screwfix Select

    For a start I've never said the cut out is there to protect the installation so please don't put words in my mouth. A fuse is an inanimate object that has no concept of 7671 or DNO's. It is simply designed to serve a purpose, no more no less.

    My point is - your argument simply does not hold water in a TT. I have shown how pointless it is to put in a second fuse. if there is a fault to earth it's just going to sit there all day, giving you the bird. It is simply left to the designer to come up with a suitable solution. If your argument is based on the fact that 7671 has no concept of the cut out such that it must be ignored, then by all means fit the s/f but why bother? What's the point? For what reason? What purpose does it serve?

    So I ask again, where is the reg or the requirement that compels you to install a s/f within 3m if it's a TT? Is this just a requirement for PME and TNS? Where does it say that? Presumably someone, somewhere must have thought this through? What are the regs concerning this situation, what are the DNO's views on this? Where is this covered in law?
     
    Last edited: May 1, 2019
  4. spinlondon

    spinlondon Screwfix Select

    Not putting word into your mouth.
    You are the one saying that we don’t need two protective devices.

    Of course it holds water.
    In a TT installation, you would rely on another method to provide Earth Fault protection.

    The purpose it serves, is it complies with BS7671 and the DNO’s will accept the installation as being safe, to allow a connection to the public supply.

    There are two Regs which require a protective device be sited within 3m.

    433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:
    (i) It is protected against fault current in accordance with the requirements stated in Section 434
    (ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also Regulation 434.2.1).

    434.2.1 Except where Regulation 434.2.2 or 434.3 applies, a device for protection against fault current may be installed other than as specified in Regulation 434.2, under the following conditions: In the part of the conductor between the point of reduction of cross-sectional area or other change and the position of the protective device there shall be no branch circuits or socket-outlets and that part of the conductor shall:
    (i) not exceed 3 m in length, and
    (ii) be installed in such a manner as to reduce the risk of fault to a minimum, and
    NOTE: This condition may be obtained, for example, by reinforcing the protection of the wiring against external influences.
    (iii) be installed in such a manner as to reduce to a minimum the risk of fire or danger to persons.
     
  5. Coloumb

    Coloumb Screwfix Select

    Right so what regulation? Where is this specified? AFAIC it's a TT distribution ctt.
     
  6. spinlondon

    spinlondon Screwfix Select

    Can you not see the two Regulations I quoted?
     
  7. Coloumb

    Coloumb Screwfix Select

    I can. So what? Where is this reg concerning "another method"?
     
    Last edited: May 1, 2019
  8. spinlondon

    spinlondon Screwfix Select

    410.3.3 In each part of an installation one or more protective measures shall be applied, taking account of the conditions of external influence.
    The following protective measures generally are permitted:
    (i) automatic disconnection of supply (Section 411)
    (ii) double or reinforced insulation (Section 412)
    (iii) electrical separation for the supply to one item of current-using equipment (Section 413)
    (iv) extra-low voltage (SELV and PELV) (Section 414).
    The protective measures applied in the installation shall be considered in the selection and erection of equipment.
    For particular installations see Regulations 410.3.4 to 410.3.9.
    NOTE: In electrical installations the most commonly used protective measure is automatic disconnection of supply.
     
  9. Coloumb

    Coloumb Screwfix Select

    So what? Your the one telling me a s/f needs to fitted for tails > 3m on a TT which is totally pointless. What document specifies this? Where is it written?
     
  10. spinlondon

    spinlondon Screwfix Select

    Regulation 433.2.2.
     
  11. Coloumb

    Coloumb Screwfix Select

    First of all it's "you need to fit a s/f" then when I show you how pointless it is on a TT it's suddenly "oh actually we need additional protection". So according to to whom? You said yourself this is a DNO requirement. So where is this fabled document that specifies this "additional protection" the DNO require? I've asked three times already and you haven't posted anything up. Personally I haven't seen it and it's funny it hasn't been mention ten posts back. If it IS a requirement then what rating 100ma, 300ma, time delayed? What? If the DNO want you to fit and RCD then don't you think they would have the decency to tell you what rating? And why don't we fit an RCD just beyond the meter for the tails now? You said yourself the tails beyond the meter come under 7671 yet they don't need "additional protection" do they? Then suddenly, oh look, longer than three meters and they do? Why? A s/f is going make zero point zero difference. The reg you keep quoting, the 3m length is for bus bar chambers and office track distribution systems, nothing to do with meter tails. And they don't mention anything about a TT either do they?

    Your just reading what ever you want into the regs and tying yourself up in knots over it. Maybe you have shares in the local DNO or some switch fuse manufactures, I dunno. But that's your trip. Like I said I've been vilified for this time after time and every time I pointed out how totally useless putting two fuses together no has ever come up with a decent, proper, solid TECHNICAL reason to do this. Maybe it's a DNO requirement? Are they infallible? Are they so brilliant they are completely beyond making any kind of mistakes? It's basis has always been based on the fact that "it has always been done like this". My tails are longer than 3m and so far the DNO police have left me alone.

    Open you eyes, take a look. Think about what I'm saying.
     
    Last edited: May 1, 2019
  12. retiredsparks

    retiredsparks Super Member

    Sen, just to save me the time...what was the unanimous agreement of all the qualified 18th regs guys and other 'experts' regarding your original question.
    cheers
    RS
     
  13. Coloumb

    Coloumb Screwfix Select

    He went back and swapped it for an RCD one anyway so it doesn't really matter any more lol
     
  14. retiredsparks

    retiredsparks Super Member

    Cheers Col.
    RS
     
  15. Bazza-spark

    Bazza-spark Screwfix Select

    As I have posted previously

    So if there is no change in CSA, method of installation, type of cable or conductor or environmental conditions the rest of the regulation does no apply and the 3m rule does not come into play.

    Kind regards
     
  16. Coloumb

    Coloumb Screwfix Select

    But that's not what he's saying Baz. He argues that 7671 totally ignores the fact that any type of protection is there at all once the meter tails go beyond three meters. It's as if the cut out simply does not exist unless you have agreement from the DNO, so of you don't have that, then well, fit a s/f. Not sure what happens if the tails are half an inch longer than three meters, but there you go.

    This is the bit he means

    "or other change and the position of the protective device"

    So there is no protective device on the tails so...

    "(i) not exceed 3 m in length"

    hence the meter tails must be less than 3m or fit a s/f.

    Try telling him otherwise.

    I guess the fudge is that if you can't agree with the DNO then you have to invoke the law to convince them otherwise. It's then up to them to prove your design is unsafe. Presuming you stay within the Zs and use SWA, I just can't see how they could.
     
    Last edited: May 1, 2019
  17. Bazza-spark

    Bazza-spark Screwfix Select

    Both regulations refer to a change in circuit conditions.

    If there is no change in circuit conditions the 3m rule does not apply, the tails can be any length. It is the change in conditions that requires the installation of the extra fuse. Hence the following formula:=

    No change in conditions = no extra fuse and no limit on length.
    Change in conditions - extra fuse and limit on length.

    Simples!

    Kind regards
     
  18. Coloumb

    Coloumb Screwfix Select

    You would think so Baz but look at the phrasing...

    434.2, under the following conditions: In the part of the conductor between the point of reduction of cross-sectional area or other change and the position of the protective device

    He say's it's OR, and, so either the change in the CSA OR the fact that there is no protective device, it doesn't need both. The cut-out simply ceases to exists and is no longer recognised as part of 7671. Believe me I've spent months trying to tell him otherwise but he's having none of it.
     
  19. Bazza-spark

    Bazza-spark Screwfix Select

    Yes Col, I have looked at the phrasing. That is why I am saying t doesn't apply.

    434.2, under the following conditions: In the part of the conductor between the point of reduction of cross-sectional area or other change and the position of the protective device

    If there is no change then good to go. The position of the protective device is irrelevant because there has been no change. The conductor is continuous, no joins, no reduction in csa, no change, so 3m rule does not apply. Cable length unaffected.

    As long as the DNO are happy for you to use their fuse = no problem.

    Kind regards
     
  20. Coloumb

    Coloumb Screwfix Select

    Your preaching to the converted mate. Well maybe he will believe you. Who knows. End of the day I'm happy to use the DNO fuse, bugger 7671 and bugger the law and put your trust in physics. That's my view. And I've done just that, twice. Been issued the building control cert, had the DNO round to install a new cut out, isolator and smart meter.

    Still waiting for the knock on the door to "help police with their enquiries".
     

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